Security light is flashing and the engine will not crank after removing and reinstalling the ignition switch

The old key had a RF chip in it for the security system disable, you will need to get a new key that is programmed to the car or try this relearn procedure.

The following is the Auto Learn Procedure:
1. Insure the ignition key is in the ON position.
2. Clear all EVO/PASSLOCK(TM) module and VCM/PCM DTCs. Record and refer any VCM/PCM DTCs to "Diagnostic System Check". See: Computers and Control Systems > Testing and Inspection
3. Momentarily rotate the ignition key to the CRANK position (Do NOT start the vehicle), then release it to the RUN position (Do NOT key OFF in the process).
4. Wait 10 minutes. Observe the SECURITY indicator. Refer to the following list for the appropriate responses:
If the PASSLOCK(TM) sensor was replaced the SECURITY indicator will flash for the 10 minutes duration.
If the EVO/PASSLOCK(TM) module was replaced, the SECURITY indicator will flash for a few seconds and then remain ON for the remainder of the 10 minutes duration.
If the VCM/PCM was replaced with a new programmed VCM/PCM and connected to the vehicle for the first time, the vehicle will start and this procedure may not be necessary.
If the replacement VCM/PCM was connected to any vehicle at any other time, even momentarily, the SECURITY indicator will flash for a few seconds and then remain ON for the 10 minutes duration.
5. The SECURITY indicator will transition from flashing to ON briefly to OFF after 10 minutes if remaining in the ignition ON position.
6. Turn the ignition switch to the OFF position and wait for 10 seconds.
7. Repeat steps 3-6 two more times. The new security code is ready to be communicated among the PASSLOCK sensor, EVO/PASSLOCK module and VCM/PCM. The new password is learned on the next ignition lock cylinder cycle from OFF to CRANK to ON (start attempt).

If this doesn't work here are the instructions on how to replace the ignition switch/key tumbler in the diagrams below. Check out the diagrams (Below). Please let us know what happens.